The invention relates to a hydraulic system for a working machine. The hydraulic system is a load sensing (LS) system and comprises a first hydraulic actuator and a first control valve for controlling the flow of hydraulic fluid from a pump to the first hydraulic actuator and for draining hydraulic fluid from the first hydraulic actuator, respectively, and a second hydraulic actuator and a second control valve for controlling the flow of hydraulic fluid from the pump to the second hydraulic actuator and for draining hydraulic fluid from the second hydraulic actuator, respectively. The hydraulic system further comprises a first circuit for providing an LS pressure for the first actuator and a second circuit for providing an LS pressure for the second actuator.
The invention can be applied on different types of hydraulic system, for example hydraulic systems for operating hydraulic cylinders for lifting an arm or tilting an implement of a wheel loader or for operating hydraulic cylinders for a dump body of an articulated hauler and/or for steering of a working machine.
Although the invention will be described with respect to a hydraulic system for a wheel loader, the application of the invention is not restricted to this particular application, but may also be used in other hydraulic systems and vehicles.
A working machine is usually provided with a bucket, container or other type of implement for digging, lifting, carrying and/or transporting a load.
For example, a wheel loader has a lift arm unit for raising and lowering an implement, such as a bucket. The lift arm unit comprises hydraulic cylinders for movement of a load arm and the implement attached to the load arm, Usually a pair of hydraulic cylinders is arranged for raising the load arm and a further hydraulic cylinder is arranged for tilting the implement relative to the load arm.
In addition, the working machine is often articulated frame-steered and has a pair of hydraulic cylinders for turning steering the working machine by pivoting a front section and a rear section of the working machine relative to each other.
The hydraulic system generally further comprises at least one hydraulic pump, which is arranged to supply hydraulic power, i.e. hydraulic flow and hydraulic pressure, to the hydraulic cylinders. The hydraulic pump is driven by a power source, such as an internal combustion engine or an electric motor. The hydraulic system of a working machine is usually a so called load sensing system (LS-system). This means that the pump that provides the actuators with hydraulic fluid receives a signal representing the current load pressure of a hydraulic cylinder in operation. The pump is then controlled to provide a pressure which is somewhat higher than the load pressure of the hydraulic cylinder.
The hydraulic pump is often a variable displacement pump that is driven by the prime mover of the working machine. If the pump is driven by an internal combustion engine, the pump is connected to a power take-off which can be located between the internal combustion engine and a transmission arrangement, such as a gear box. The transmission arrangement is in turn connected to e.g. wheels of the working machine for the propulsion thereof.
When driving a hydraulic cylinder in an LS-system, hydraulic oil is supplied by the pump and the flow of hydraulic oil from the pump is directed by an inlet valve to one side of the hydraulic cylinder and the flow of hydraulic oil from the other side of the hydraulic cylinder is drained to tank by an outlet valve.
The pump pressure is the LS pressure (representing the actual load pressure of the actuator) plus a margin pressure. When the pump is used for several functions the pump is controlled by the highest LS pressure and a certain margin pressure. A disadvantage with such a system is however that the pump is always supplying hydraulic oil with the same margin pressure. For example, different functions may require different margin pressures. In a case where the LS pressure of a function that requires a lower margin pressure determines the pump pressure, the losses will be unnecessarily high since the pressure drop over the control valve will correspond to the margin pressure.
It is desirable to provide a hydraulic system, by which system the energy losses and thereby the fuel consumption can be reduced.
The invention, according to an aspect thereof, is based on the insight that by the provision of a hydraulic system where at least one of the first and second circuits comprises an offset valve for changing the LS pressure before providing the LS pressure to the pump, different margin pressures can be obtained for different functions.
For example, in a wheel loader the steering hydraulics needs a higher margin pressure than the working hydraulics (such as lift, tilt, etc.). By providing different margin pressures the working hydraulics can be driven more efficiently when an LS pressure of this function determines the pump pressure. The energy losses can be reduced due to the fact that the pressure drop over the control valve is decreased.
According to one embodiment of the invention, the hydraulic system comprises an offset valve arranged for increasing the LS pressure. Hereby, the first actuator can be driven while using a higher effective margin pressure. The pump may be controlled to provide a pump pressure based on the LS pressure and a predetermined margin pressure By increasing the LS pressure, the pump pressure will be higher and thus the effective margin pressure will be higher than the predetermined margin pressure.
According to a further embodiment of the invention, the offset valve comprises a first port for connection to the incoming LS pressure and a second port for connection to a pressure source having higher pressure than the incoming LS pressure, and a port for providing an increased LS pressure, and a spool for selecting between a first state, where the first port is closed and the second port is opened, and a second state, where the first port is opened and the second port is closed, wherein the offset valve further comprises a spring arranged to apply a force on the spool in a first direction towards the first state, and the hydraulic system has a means for applying the incoming LS pressure to the spool for creating a force in said first direction towards the first state, and a means for applying the increased LS pressure to the spool for creating a force in a second direction towards the second state. Hereby, a positive offset of the LS pressure and thereby an increased effective margin pressure can be achieved in a non-complicated and robust way.
According to one embodiment of the invention, the hydraulic system comprises an offset valve arranged for decreasing the LS pressure. Hereby, the first actuator can be driven while using a lower effective margin pressure. The pump may be controlled to provide a pump pressure based on the LS pressure and a predetermined margin pressure. By decreasing the LS pressure, the pump pressure will be lower and thus the effective margin pressure will be lower than the predetermined margin pressure.
According to a further embodiment of the invention, the offset valve comprises a first port for connection to the incoming LS pressure and a second port for connection to a pressure source having lower pressure than the incoming LS pressure, and a port for providing an decreased LS pressure, and a spool for selecting between a first state, where the first port is closed and the second port is opened, and a second state, where the first port is opened and the second port is closed, wherein the offset valve further comprises a spring arranged to apply a force on the spool in a first direction towards the first state, and the hydraulic system has a means for applying the decreased LS pressure to the spool for creating a force in the first direction towards the first state, and a means for applying the incoming LS pressure to the spool for creating a force in a second direction towards the second state. Hereby, a negative offset of the LS pressure and thereby a decreased effective margin pressure can be achieved in a non-complicated and robust way.
According to a further aspect, the invention relates to a working machine. The same advantages as discussed above with reference to the hydraulic system can be reached by the working machine according to the invention.
Further advantages and advantageous features of the invention are disclosed in the following description and in the dependent claims.